From the aforementioned copending applications and the literature generally in the flue gas cleaning arts, it should be apparent that the desulfurization of a flue gas, especially from a fossil-fuel power plant combustion system, may involve the introduction of a calcium compound, generally lime or calcium carbonate so that the sulfur or sulfur compounds will eventually be removed by an absorption of sulfur oxide and/or transformation thereof to calcium sulfate, for example, which can collect in the sludge of the slurry at the sump of the scrubbing tower.
It is known, therefore to desulfurize such flue gases by scrubbing them in a scrubbing tower with a lime or calcium carbonate-containing scrubbing suspension and indeed it is also known to introduce oxygen into the tower in the form of air, technical grade oxygen or oxygen enriched air to maximize the recovery of the calcium sulfate in the sump of the tower.
The sump product is separated into the recovered solids and a scrubbing suspension which is recycled and at least a portion of the sump product can be delivered to a thickener from which a calcium sulfate dihydrate sludge or slurry is withdrawn.
For convenience, the term "lime" will be used herein to refer to any sulfur-binding calcium compound, including calcium oxide, calcium hydroxide and calcium carbonate and any compounds which may tend to form these sulfur-binding compounds or which can yield a calcium sulfate dihydrate upon reaction with flue gas in a scrubbing solution.
In the conventional processes of the type described, the calcium-sulfur compound which predominates is the calcium sulfate dihydrate although some not insignificant amount of calcium sulfite may be present therein.
The presence of even small quantities of calcium sulfite in the sludge or slurry creates problems especially when the sludge or slurry is to be used in the production of plasterboard or like gypsum building materials in which the gypsum may be laminated with cardboard or the like.
It has, therefore, been proposed to transform the calcium sulfate dihydrate sludge to a gypsum alpha semihydrate or hemihydrate, a product which can be utilized without difficulty in the production of plasterboard.
When the calcium sulfate dihydrate was to be used directly in the production of plasterboard, it required calcination and generally the quality of the resulting product did not suffice. It appears that the particle size or granulation of the individual calcium sulfate dihydrate was such that dewatering and/or drying was made difficult and the relevant chemical reaction could not be effected reliably. Attempts were made to overcome these disadvantages by varying the processing parameters without significant success and indeed success can hardly have been expected because the development of a narrow range of effective processing parameters including treatment times, temperatures, flow rates and the like could not be expected to be meaningful because of the intrinsic variation in the calcium sulfate products which were treated.